Variable pulley transmissions for transferring torque from an input or drive shaft to an output or driven shaft have been used for some time. In these transmissions, a first pulley constructed of a pair of flanges is mounted on the input shaft such that at least one of its flanges is axially movable with respect to its other flange. A second, similarly constructed and adjustable pulley is mounted on the output shaft. A flexible belt connects the two pulleys to transfer torque therebetween when the input shaft is driven. As the effective diameter of one pulley is changed and, simultaneously, the effective diameter of the other pulley is changed in the opposite direction, the drive ratio between the input and output shafts is adjusted in a smooth, continuous manner.
Automative engineers have long recognized that the maximum operating efficiency of the engine could be achieved if the transmission could be controlled by adjusting to different loads and speed ratios, such that the engine is maintained and operated at its maximum efficiency operating conditions. This is not possible when a conventional geared transmission is teamed with an engine. In the conventional geared transmission, the drive ratio is adjusted in discrete steps, rather than continuously. Accordingly, efforts have been directed to the use of a continuously variable transmission (CVT) of the type described above. These efforts have resulted in the production and marketing in Europe of the DAF passenger car, using a flexible, continuous rubber belt to drivingly interconnect the pulleys. Such a belt is subject to wear by reason of the torque it must handle and operates under severe temperature, vibration and other adverse conditions. To improve the belt life, efforts have been channeled to produce a flexible belt of metal, and some of these efforts are described in the patent literature.
Flexible metal belts for use with CVTs are generally of two varieties, those referred to as "push" belts and those referred to as "pull" belts. An example of a push belt is described in Van Doorne et al U.S. Pat. No. 3,720,113 and an example of a pull belt is described in Cole, Jr. et al U.S. Pat. No. 4,313,730. The Van Doorne et al belt comprises an endless carrier constructed of a plurality of nested metal bands and an endless array of load blocks longitudinally movable along the carrier. Each block has edge surfaces frictionally engaging the pulley flanges of a pulley transmission to transmit torque between the pulleys. The pull belt of Cole, Jr. et al utilizes an endless chain as the carrier, the sets of links of which are pivotably interconnected by pivot means, shown as round pins. Generally trapezoidal (when viewed from the front) load blocks encircle the links; however, the load blocks are constrained against longitudinal movement along the chain by the pivot means.
The push belt as described is relatively expensive to manufacture because the nested carrier bands are precisely matched to each other. Such a belt must be installed and/or replaced as a complete, endless loop, and thus disassembly of parts of the pulley transmission is required, not only for the initial assembly, but also for replacement due to failure of one or more load blocks or one or more of its carrier bands.
The pull belt offers a less expensive alternative to the push belt. No precise matching of carrier parts is required. The belt can be assembled with a finite length, positioned around the pulleys, and the ends of the belt then are connected by a pivot member to make an endless belt. Thus disassembly of the pulleys is not required either for initial installation or for replacement of a belt.
Load blocks have a tendency to rock or tilt with respect to the carrier, especially when entering the pulley. Thus the edge surfaces may engage the pulley flanges slightly askew to a radial line. When the block's "window" or "windows", i.e., the opening or openings in which the carrier is located, are made with square defining edges, as is customary when the blocks are stamped from sheet metal, the tilting of the blocks causes the top and/or the bottom window defining edges to dig into and damage the carrier, thus weakening the carrier and seriously affecting its ability to transmit torque. The damage to the carrier leads to its premature failure. One approach to solving this problem and not yet proven is to make the top and bottom window defining surfaces from front to back slightly round or arcuate. This procedure adds to the manufacturing costs of the belt.
Load blocks for use with either carrier system have been constructed with generally flat, planar, pulley-flange-engaging edge surfaces. These surfaces are, at times, joined to the top and bottom surfaces of the blocks by curved surfaces of small radii which have no effect on block-pulley contact. Load blocks, during their torque transmitting operation, are pulled inwardly of each pulley and are thus subjected to transversely applied compressive loads which unduly stresses the blocks and which can lead to their failure.